Improve llvm-mc disassembler mode and refactor ARM tests to use it

This allows "llvm-mc -disassemble" to accept two new features:
  + Using comma as a byte separator
  + Grouping bytes with '[' and ']' pairs.

The behaviour outside a [...] group is unchanged. But within the group once
llvm-mc encounters a true error, it stops rather than trying to resynchronise
the stream at the next byte. This is more useful for disassembly tests, where
we have an almost-instruction in mind and don't care what the misaligned
interpretation would be. Particularly if it means llvm-mc won't actually see
the next intended almost-instruction.

As a side effect, this means llvm-mc can disassemble its own -show-encoding
output if copy-pasted.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186661 91177308-0d34-0410-b5e6-96231b3b80d8
ARM: remove invalid invalid tests

The tests were checking for barriers which the ARM ARM says they must execute
as a full system DMB/DSB, rather than that they're UNDEFINED and LLVM does in
fact represent them.

The tests happened to be passing because they were using a non-versioned ARM
triple which didn't have *any* DMB/DSB instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186662 91177308-0d34-0410-b5e6-96231b3b80d8
ARM: delete two tests now integrated into the larger files

Somehow forgot to git rm these two files. I believe I left the remaining
invalid* tests intentionally, though whether my reasons were sound is a
different matter.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186663 91177308-0d34-0410-b5e6-96231b3b80d8
Fix another assert failure very similar to PR16651's test case. This
test case came from Benjamin and found the parallel bug in the vector
promotion code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186666 91177308-0d34-0410-b5e6-96231b3b80d8
Cleanup the stats counters for the new implementation. These actually
count the right things and have the right names.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186667 91177308-0d34-0410-b5e6-96231b3b80d8